What is the Horizontal Gaze Nystagmus Test?

What is the Horizontal Gaze Nystagmus Test?

Posted By
Jonathan Blecher, P.A.

The Horizontal Gaze Nystagmus (HGN) test is a field sobriety test used
by law enforcement to determine whether or not a suspected driver is under
the influence of alcohol. The test involves an officer asking a driver
to follow some stimulus, usually a pen, with their eyes to the left and
right. The officer will then determine when the eye begins involuntarily
jerking, or exhibiting nystagmus. While all eyes experience this involuntary
twitch when looking at a significant angle, those with higher blood alcohol
levels begin twitching sooner. According to the National Highway Transportation
Safety Administration, HGN tests are 77% reliable in determining whether
or not a driver’s BAC is above .10%.

How does an officer determine nystagmus?

A HGN test cannot confirm or deny the amount of alcohol that a suspected
driver may have in their system, but indicates to the police officer that
there may be some level of impairment present.

When testing HGN, an officer will check for the following questions:

Are the eyes moving smoothly from one side to the other or noticeably jerking?

Are the eyes jerking when they have moved as far as possible to one side?

Does the eye begin jerking before it has reached a 45-degree angle?

There are a few factors that can affect the outcome of an HGN test. A suspect
that is wearing eyeglasses may make it more difficult for the officer
to see the angle at which the suspect’s eyes begin to twitch. Additionally,
if the HGN test is not administered in a well-lit area, the officer may
not be able to tell when nystagmus occurs. Suspects should not be facing
headlights or the lights of a police vehicle since this can cause nystagmus
without the help of alcohol. If any of these apply in a suspected driver’s
case, the results of the HGN test may be inadmissible in court.

There are other defenses to HGN tests, including the ability of the officer
to determine a 45-degree angle, the presence of prescription drugs in
the body of the accused, the presence of a head injury, and more. Working
with a DUI defense attorney will be the best bet in clarifying the cause
of nystagmus and whether the results of this test can be permitted in court.

As a
Miami DUI attorney, I will work tirelessly to fight the evidence presented against you. Call
(305) 707-0036 today to begin discussing your legal options.